(a) Technical Field
The present disclosure relates to an electromechanical brake for a vehicle, more particularly, to an electromechanical brake capable of exhibiting stable braking performance even when a system change occurs such as a disturbance applied to a pad or abrasion of the pad.
(b) Description of the Related Art
In general, a brake device for a vehicle is a device that generates braking force for decelerating or stopping a moving vehicle or maintaining the vehicle in a stopped state, and braking is carried out while kinetic energy of the vehicle is converted into thermal energy by mechanical friction when the vehicle decelerates and frictional heat is radiated into the atmosphere.
As the brake device for a vehicle, there are a drum type hydraulic brake, a disc type hydraulic brake, and the like, and the disc type hydraulic brake obtains braking force by strongly pressing friction pads (brake pads) against both surfaces of a disc, which rotates together with a wheel, instead of using a drum.
However, the hydraulic brake has a complicated structure because the hydraulic brake requires mechanical elements connected to a brake pedal in front of a driver seat, hydraulic piping, elements for controlling hydraulic pressure, and the like, and therefore, an electromechanical brake (EMB) has been developed and used to simplify a configuration of the brake device.
The electromechanical brake has been developed to be used as an electronic parking brake (EPB), but recently, a field of the electromechanical brake has been expanded such that the electromechanical brake is used as a main brake that is substituted for the hydraulic brake in the related art.
Unlike the typical hydraulic brake, the electromechanical brake refers to a brake that obtains braking force by pressing a friction pad by using a mechanical mechanism driven by an electric motor.
The typical electromechanical brake has an actuator including the electric motor that rotates forward and backward in order to perform the braking operation (press the friction pad) and release the braking operation (reduce pressure), and the electromechanical brake operates to press the friction pad using rotational force of the motor so that the friction pad presses the disc (causes friction with the disc) at the time of performing the braking operation.
Compared to the hydraulic brake, the electromechanical brake has a simple structure and a high response speed, and may be more precisely controlled, thereby improving braking safety.
The electromechanical brake is advantageous because braking force is easily controlled, and essentially used to implement a brake-by-wire (BBW) system.
Meanwhile, the electromechanical brake converts rotational force of the actuator into translational force so as to press the friction pad against the disc, and allow the friction pad to cause friction with the disc in a state in which the friction pad presses the disc, thereby generating braking force.
In this case, a spindle (also referred to as a screw) rotates, but a piston, which pushes the friction pad, does not rotate but needs to move straight, and as a result, there is required a structure which supports the piston such that the piston does not rotate but moves straight forward and backward in an axial direction.
In general, there is applied a structure in which a protrusion is formed on a rigid body of the friction pad, and a groove into which the protrusion is inserted is provided in the piston, such that the protrusion is inserted into and caught by the groove, and as a result, the piston cannot be rotated by the friction pad when the spindle rotates.
Alternatively, because precise control is not required in the case of an electromechanical brake for parking a vehicle, that is, in the case of the EPB instead of a typical electromechanical brake for braking a vehicle, a groove is formed in a surface of the piston which abuts on the friction pad, such that the rotation of the piston is prevented by increasing frictional force between the friction pad and the piston.
In accordance with regulations, the EPB needs to generate a predetermined or higher level of braking force even when the vehicle travels as well as when the vehicle is parked, and the EPB needs to be robustly designed so that stable performance may be outputted even though there occurs a system change such as abrasion of the friction pad and a disturbance transmitted when the vehicle travels.
In particular, in a case in which the electromechanical brake is applied as the typical main brake for braking a vehicle, it is necessary to minimize an influence of disturbance in order to precisely control braking force.
As known, in the case of the disc brake that generates braking force by using friction between the disc and the friction pad, the friction pad shifts toward the disc as the friction pad is abraded, and deflection of an angle occurs due to non-uniform abrasion of the disc and the pad as durability thereof is changed.
Because the pad is installed to have a predetermined clearance in order to reduce residual braking force (braking drag) and reduce vibration and noise when the braking operation is released, the pad moves in a rotational direction when the pad causes friction with the disc at the time of performing the braking operation.
In the case of a structure that restricts and supports the piston so as to prevent the rotation of the piston by using the protrusion of the pad, an additional rotation occurs about an axis of the protrusion at the time of performing the braking operation.
In a case in which the rotation of the piston is restricted by the pad like the related art, the movement of the pad in various directions is transmitted to components of the electromechanical brake through the piston as it is, and as a result, precise control cannot be carried out. The spindle and the piston are bent and axes are misaligned, and as a result, there are problems in that efficiency deteriorates and thus durability deteriorates.
It is also impossible to control braking force in a case in which the protrusion of the pad is separated from the piston, and in a case in which the protrusion of the pad is securely fixed to the groove of the piston in an interference-fit manner in order to prevent the problem, there is a problem in that maintainability deteriorates.
FIGS. 1A and 1B (RELATED ART) are views for explaining a disturbance that occurs on a friction pad of an electromechanical brake in the related art, and as illustrated in FIG. 1A, a pad 1 moves in various directions due to abrasion of the pad 1, an increase in pressure at the time of performing the braking operation, and a decrease in pressure at the time of releasing the braking operation, and deflection of an angle may occur due to non-uniform abrasion of a disc (not illustrated) and the pad 1.
As illustrated in FIG. 1B, the pad 1 may move leftward and rightward as the disc rotates, and the pad may rotate due to rotational force of a spindle.
Misalignment of axes in the electromechanical brake in the related art will be described in more detail with reference to FIGS. 2A and 2B (RELATED ART).
Reference numeral 2 indicates a contact surface of the friction pad 1 which comes into contact with the piston, reference numeral 3 indicates a protrusion of the friction pad 1, and reference numeral 4 virtually indicates a shape and a position of a groove of the piston into which the protrusion 3 of the friction pad 1 is inserted.
Reference numeral 5 indicates a torque member.
In an initial state, an axial center of the spindle and an axial center of the piston coincide with each other as illustrated in FIG. 2A, and as illustrated in FIG. 2B, misalignment of the axes occurs due to the rotation of the pad 1 at the time of performing the braking operation when the vehicle is stopped.
That is, the axial center of the piston and the axial center of the spindle may be misaligned.
The misalignment of the axes may become severe due to the rotation and the leftward and rightward movement of the pad 1 at the time of performing the braking operation while the vehicle travels.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.